My 95 with a 3.4 just started misfiring badly once the motor warms, and will bog and shake under any kind of load. It runs smooth as usual until it gets up to operating temp, and I shut it off and restart a couple of times, like if Im running around town with normal useage. I've checked all the wires, and replaced them about a year ago. Checked sensors for wiring issues.. Nothing is crossed. No codes.. Coil packs may be original as well as ICM. Thanks in advance for any help.

 

Get yourself some brake cleaner, spray all your vacuum lines, see if theres a leak. that can cause a miss. your fuel pump may also be bad, and when it gets warm its even worse. Smart money is on one of those two, so check em out.

 

Thanks JakeYeah.
I noticed this morning it began to miss when the motor started to get hot, like at idle with AC off, and just before the cooling fans turn on. Hard to tell what the temp is on the gauge at least in regards to accuracy, but the needle is approximately in the middle of the gauge. Gorn, if you're out there what do you think!?

 

I offer this tidbit:

When the engine is cold, the Alpha Dog sensor is the coolant temp sensor. The PCM uses coolant temp info as the main input for determining injector pulsewidth (amount of fuel injected). Oxygen sensor data is ignored.

As the engine warms up and the oxygen sensor(s) get hot, they begin to provide reliable data and the PCM transitions to using *them* as the primary input.

Warmup time takes several minutes (on cars like a 95 3.4 that doesn't have heated oxygen sensors) so exactly how long does yor car run well before it starts misfiring?

Also, oxygen sensors can work very poorly and not set codes. In fact the condiions for setting an O2 sensor code are pretty drastic.

Can you scan the O2 sensor operation?

 

^^^ agree check temp sensor, also 02 sensors for fouling, sounds sensor related. maybe check wires to knock sensor. this should be same for your temp sensor 4th Gen LT1 F-body Tech Articles

 

Thanks guys. Well it looks like Im losing coolant as well, with no evidence of any leaks outside the motor. Im filling the coolant reservoir twice a week now.. Think I may have a blown head gasket and coolant is leaking in the cylinder and burning off, and possibly causing this miss as well. The car runs great until it gets hot, like normal temp to warmer, then misses. But lately the cold starts have been rough now too, with an occasional cough and slight backfire, then the car smooths out until it gets hot etc. Would that be related to a blown gasket? Or a sensor problem?
Im not getting any smoke out the tailpipe.. And just changed the thermostat before the miss began as well. Thanks a bunch.

 

could be the intake gasket leaking. your filling the radiator and not the reservoir right?

 

I usually fill both. I regret to say the car use's almost a gallon of water/coolant a week, with an average of 150 city driving miles.

Well the car is still having this rough idle and poor acceleration issue. The only difference now is that it starts to act up even at cold starts. In fact most starts are sluggish. No codes, and nothing in memory. I've replaced the coolant tempereture sensor, the 24x crank position sensor, and the cam position sensor. No change. I've checked all the vacuum lines to the MAF etc. Plug wires, sensor connectors etc. I unplugged the MAF while the car was running and it cut right out and didn't restart until I plugged it back in. I unplugged the O2 sensors and it made no difference in the running. (?) Didn't even get a light. But the SECURITY light came on, just after the crank sensor install and ecm reset. That's never happened before, and I usually try to reset after a sensor install and never had the SECURITY light come on. It's staying on now.
Sorry for the late reply. Been working on it.. Thanks for anymore help.

 

Your going to have to track down where the coolant is going. Based on what you have said so far it sounds to me like you have a blown head gasket. When the car warms up the system starts to build pressure. A small spray of coolant into the cylinder will short the plug just like it was flooded. (The "electrical energy" travels through the coolant and never makes a spark)

I am not saying that is what it is but it fits for everything you are saying.

 

^^^ agree better do a compression test on all cylinders to see if you can find for sure if its a head gasket